If anyone knows why this is happening, I'd greatly appreciate any advice.

I understand that one can play RTW multiplayer with mods installed, provided that each player is uisng identical mods.

I play RTW using a combination of my mod, Vanilla Balance Mod, and a personal mini-mod which I have specially made to be installed on top of it. This simply effects a couple of texture changes - a new skin for spartan hoplites and different faction symbols and banners for a couple of factions. Both mods are installed by running NSIS scripted setup programs.

I have two PCs on my home network, an aged laptop running Windows XP and a desktop running Vista (user account control turned off). A clean installation of RTW was installed on both machines, with no traces of any previous installs already present. At this point, the two PCs can successfully play Rome over LAN.

Next, BugFixer is installed on both machines. Again, LAN games function.

Then, Vanilla Balance Mod is installed on both. Still, LAN games function.

Finally, my personal mini-mod is installed. Suddenly, LAN games cease to work, with the system claiming that the host and joiner are running incompatible versions of RTW.

All of the modifications have been installed in the same order, from the same setup program (literally carried between computers on a USB drive), over clean installs, with absolutely NO manual editing (or even playing) done in the mean time.

What on earth is causing the incompatibility?